A development of InGaZnO4 (a-IGZO) thin film transistors (TFT) which, in an amorphous state, have higher mobility than a-Si has promoted a research and development in an effort to make oxide semiconductors practicable all over the world. However, almost all of these oxide semiconductors have been an n-type oxide semiconductor which electrons serve as a carrier.
If a p-type oxide semiconductor which properties are comparable with that of the n-type oxide semiconductor becomes available, the p-type oxide semiconductor can be combined with the n-type oxide semiconductor to form a p-n junction which results in, for example, a diode, an optical sensor, a solar cell, a LED, and a bipolar transistor. The oxide semiconductor can be made into a wide bandgap semiconductor, which allows a device including the semiconductor to be transparent.
In an active matrix organic EL display, a basic driving circuit is a 2T1C circuit as shown in FIG. 7. In this case, a driving transistor (field effect transistor 20) which is an n-type transistor results in a so-called source follower connection. Thus, a time-dependent change (especially voltage rise) of organic EL device properties causes an operating point of the driving transistor to move to another operating point at different gate voltage, which shortens a half-life of the display. This is the reason why an AM-OLED (active matrix organic EL display) has not been practicable yet which uses a-IGZO TFT having high mobility as a backplane, and at present, a p-type LTPS-TFT (low temperature polysilicon thin film transistor) is solely employed. As a result, a high-performance p-type oxide semiconductor s again strongly desired.
It has been known from 1950s that a Cu2O crystal exhibits p-type electrical conductivity (see, for example, NPL 1). This crystal is based on an O—Cu—O dumbbell structure, and, in the structure, a hybrid orbital of Cu 3d and O 2p constitutes the top of a valence band. An oxygen-excess nonstoichiometry results in a hole in the foregoing valence band, which leads to p-type conductivity.
Examples of the crystal based on the dumbbell structure include a delafossite crystal represented by the following formula: CuMO2 (where M=Al, Ga, or In) and a SrCu2O2 crystal. Oxides thereof should have high crystallinity in order to exhibit p-type electrical conductivity. Thus, it is only CuAlO2, CuInO2, and SrCu2O2 that is actually reported to exhibit p-type electrical conductivity (see, for example, NPLs 2 to 4).
One reason why it is difficult to exhibit p-type electrical conductivity is that the valence of Cu and the amount of oxygen cannot be easily controlled. A Cu2+-containing crystal phase such as CuO, SrCuO2, and SrCu2O3 is often contaminated in an effort to form a single phase film composed of a Cu+-containing oxide which has excellent crystallinity. Such contaminated film cannot exhibit excellent p-type electrical conductivity and cannot be easily controlled in properties. This means that properties such as carrier concentration and carrier mobility cannot be optimized when these p-type oxide materials are used for an active layer in a semiconductor device.
In addition, a delafossite oxide containing monovalent Cu or Ag has been proposed (see PTL 1). However, the above proposed technology requires a heat treatment at high temperature of 500° C. or more, which is not practical.
A p-type electrical conductive thin film containing crystalline SrCu2O2 has been proposed (see PTL 2). In the above proposed technology, the thin film can be formed at relatively low temperature of 300° C. However, the thin film can only exhibit electrical conductivity of up to 4.8×10−2 Scm−1, which is insufficient. The electrical conductivity also cannot be appropriately controlled.
That is, the above proposed technologies are neither capable of producing the p-type oxide in a practical manner nor capable of resulting in the p-oxide material exhibiting appropriately controlled and sufficient electrical conductivity.
A TFT has been proposed using, as an active layer, a p-type oxide material which has a delafossite crystal structure containing monovalent Cu or Ag (see PTL 3).
However, the above proposed technology has not disclosed sufficient information with regard to, for example, material properties of an active layer, a method for producing the active layer, and transistor properties.
A TFT has also been proposed using, as an active layer, a Cu2O crystal (see NPLs 5 and 6). However, the above proposed technologies could not achieve a practically usable level with regard to, for example, the electron field effect mobility and the on-off ratio of the TFT because the active layer could not be sufficiently controlled in properties.
That is, the above proposed technologies neither capable of easily controlling various properties such as carrier concentration of the p-type oxide material nor achieving suitable properties for being used in a device.
In conclusion, no practical and useful p-type oxide material has been found.
Accordingly, there is still a need to provide a p-type oxide which properties are comparable with that of n-type oxides, a p-type oxide-producing composition for producing the p-type oxide, a method for producing the p-type oxide, a semiconductor device using the p-type oxide in an active layer, a display device having the semiconductor device, an image display apparatus using the display device, and a system including the image display apparatus.